Recently, due to the progress the epitaxial layer growth technology the GaN based devices are widely used in the fabrication of photo-detectors for UV region of spectrum. This is related to the GaN direct electron transitions and large band gap value of 3,39 eV. Such photo-detectors are widely used in astronomy, for flame detection, in the system of water and air pollution monitoring [1]. AlxGa1-xN based photodetectors have a large perspective for a series of medical and bilogical devices and on various fields including military [2-4].  Electrical and photoelectrical properties of a photo-detector with nSi-GaN heterojunction were studied. The heterojuncion was fabricated by depositing of 20 µm epitaxial GaN layer by using HVPE method in a silica tube at the atmospheric pressure on Si support having the electrical resistivity of 4,5 Ω.m. The hydrogen double purified through a palladium filter was used as a transport agent.  The ammonia, hydrogen chloride, (99,999%) Ga were used as precursors. The ammonia and HCl flows in gallium lines were dissolved in 200 smlpm of H2.  The total hydrogen consumption was of 4,8 slpm and of ammonia of 2,4 slpm. The HCl consumtion at GaN layer deposition was of about 5 slmp during etching and ~50…100 slpm during the layer growth. The temperature profile in the reactor was provided by a resistive heater. The gallium source temperature was constant close to 1250-C. For nSi-nGaN structure the ohmic contacts were provided by siver grating as a frontal contact to GaN and aluminum as a bottom contact ti Si. Both types of contact were deposited by thermal evaporation in vacuum and annealed in H2 ambiance at 500oC.  The studies of  Al-Si-GaN-Ag structure  I=f(U) and C=f(U) dependencies was determined the diffusion potential at 300K which makes 0,85 V. The Al-nSi-nGaN-Ag structure photosensitivity spectral dependence is given in Fig.1. The photosensitivity low value in the UV region of spectrum is determined by the high value of GaN layer thickness (20 -m). The photons energy of band to band electron transitions estimated from the photosensitivity spectral dependence is of 3,26 eV which is lower than GaN band gap which could be related to high value of charge carrier concentration in GaN layer. The studied nSi-nGaN structure photosensitivity range covers wavelength region from 290 nm to 1130 nm. Therefore it could be used as a photodetector for this region of spectrum.